Species response to changing light and water availability along a gulf coast salt marsh forest gradient

Background/Question/Methods

In 2004, Hurricane Ivan brought high winds uprooting trees and altering canopy cover along the gulf coast.  In 2005, Hurricane Katrina produced a strong storm surge altering soil salinity.  Decreased canopy coverage and higher salinity levels may have altered plant distribution at Weeks Bay Estuarine Research Reserve in Alabama.  Both Baccharis halimifolia, groundsel bush, and Cladium mariscus, sawgrass, distributions expanded into the forested portions of a permanent transect after the hurricanes, suggesting that changes in light resulting from openings created by tree mortality in response to wind or salinity may have favored these species.  We compared water relations of these two species along the permanent transect to a third species, Myrica cerifera, wax myrtle, whose distribution did not change post hurricane to understand how physiological response to changing conditions along the transect might explain the change in post-hurricane distribution of sawgrass and groundsel bush.  We measured diurnal stomatal conductance and xylem water potential at five different sites along the transect which spans a range of soil water availability and light.  The portions of the transect sampled were freshwater marsh, sparse ecotone, dense ecotone, hydric forest, and mesic forest in July of 2008.  We collected branches and determined specific leaf weight (SLW) for all three species, and xylem density and vessel anatomy for groundsel bush and wax myrtle. 

Results/Conclusions

Wax myrtle SLW was about 100 g/m² for all sites; however, SLW of sawgrass decreased significantly from 225 g/m² to 150 g/m² between the freshwater and dense ecotone sites. Groundsel bush displayed lower midday xylem water potential than wax myrtle across all sites although SLW did not vary significantly across all sites suggesting that it is able to sustain lower midday water potential without limiting leaf area for photosynthesis, but may require higher light levels.  Groundsel bush stomatal conductance was somewhat higher than wax myrtle across all sites.  Sawgrass xylem water potential was always more negative than all other species across all sites, and stomatal conductance was usually lower than the other two co-occurring species.  This suggests that light limitation, not water limitation may exclude sawgrass from forested areas.  Xylem density and vessel wall to lumen ratios did not differ between groundsel bush and wax myrtle across or within sites. These results may provide a glimpse of the future community structure of gulf coast wetlands since increasing sea levels will change salinity gradients resulting in canopy changes that favor a different species assemblage.